Many types of printing systems include one or more printheads that have arrays of dot forming elements that are controlled to make marks of particular sizes, colors, or densities in particular locations on the recording medium in order to print the desired image. In some types of printing systems the array(s) of dot forming elements extends across the width of the page, and the image can be printed one line at a time, as the recording medium is moved relative to the printhead. Alternatively, in a carriage printing system (whether for desktop printers, large area plotters, etc.) the printhead or printheads are mounted on a carriage that is moved past the recording medium in a carriage scan direction as the dot forming elements are actuated to make a swath of dots. At the end of the swath, the carriage is stopped, printing is temporarily halted and the recording medium is advanced. Then another swath is printed, so that the image is formed swath by swath.
In an inkjet printer, the dot forming elements are also called drop ejectors. A drop ejector includes a nozzle and a drop forming mechanism (such as a resistive heater for thermal inkjet, or a piezoelectric device for piezoelectric inkjet) in order to generate pressure within an ink-filled chamber and eject ink from the nozzle. In page-width inkjet printers as well as in carriage inkjet printers, the printhead and the recording medium are moved relative to one another as drops are ejected in order to form the image. When drops are ejected from the nozzle toward the recording medium, a major portion of the ink is contained at the head of the drop, i.e. the leading portion of the drop. A lesser portion of the ink is contained in the tail of the drop, which initially takes the form of a narrower column of ink trailing the head of the drop. As the drop continues to fly toward the recording medium, the head typically moves at higher velocity and breaks off from the tail to form a main drop. The tail typically breaks up to form one or more smaller satellite drops that hit the recording medium after the main drop, because they are slower than the main drop. Because the recording medium is being moved with respect to the printhead, the slower satellite drops land at a different position than the main drop. In addition, there can be an angular difference in the trajectories of the main drop and the satellite drops, leading to further displacement, which can be additive to or subtractive from the velocity-dependent separation, depending on relative motion direction of printhead and recording medium. In a bi-directional print mode in a carriage printer, the satellite drops can land on one side of the main drop during a right-to-left printing pass, and on the other side of the main drop during a left-to-right printing pass. Thus satellite spots can cause printing defects including broadened vertical line width, fuzzy vertical line edges, and apparent jaggedness between portions of a vertical line that are printed by successive swaths printed in different directions.
In the prior art attempts have been made to reduce print defects due to satellites by reducing print speed, changing ink formulation to modify properties such as surface tension, or refining pulse optimization. Other attempts have included using an asymmetric nozzle to steer satellite drops so that they tend to land closer to the main drop, when printing in a preferred direction. However, with such a nozzle geometry, satellite caused defects are compounded when printing in the opposite direction.
What is needed is an improved inkjet printing device that is capable of printing at full speed, is compatible with a wide range of inks and driving conditions. In addition, what is needed for carriage printers having bi-directional print modes is an inkjet printing device that reduces satellite printing defects for both left-to-right and right-to-left printing swaths.